Magnetic-field induced quantum phase transition and critical behavior in a gapped spin system TlCuCl3

Abstract Magnetization measurements were performed on TlCuCl 3 with gapped ground state. The critical density and the magnetic phase diagram were obtained. The interacting constant was obtained as U / k B = 313 K . The experimental phase boundary for T 5 K agrees perfectly with the magnon BEC theory based on the Hartree–Fock approximation with realistic dispersion relations and U / k B = 320 K . The exponent φ obtained with all the data points for T 5 K is φ = 1.99 , which is somewhat larger than theoretical exponent φ BEC = 3 2 . However, it was found that the exponent converges at φ BEC = 3 2 with decreasing fitting window.